Finally we come to part 3 of the exposure triangle, ISO. ISO is the abbreviation for the International Standards Organisation and for those that not only remember film but remember film from a long time ago it was once referred to as ASA or American Standards Association.  So what is ISO? ISO is the measurement of the sensitivity of the light capture medium i.e, film or digital sensor; basically it's how fast the capture medium reacts to the light hitting it.

Lets use film to describe ISO since the principle is the same but it is very easy to demonstrate when talking about film.  The emulsion of film is made up of thousands of grains of silver halides.  Silver is light reactive and will turn black when exposed to light, but how fast the silver reacts is determined by the size of the grains.  Very fine grains of silver will require more light to react while bigger chunkier grains of silver can react much faster. So the clever people at Kodak, Fuji, Ilford & Agfa made film with varying degrees of grain size.  Film could typically be found in the following speeds;

25, 50, 100, 125, 160, 200, 400, 800, 1600, 3200

Thanks to TigersharkBAS over at Mactalk has taken a look at the Nikon GPS Unit.

With iPhoto 09 and Aperture 2 introducing thrusting the idea of "geo tagging" into the mainstream, camera manufacturers are also responding to the desire for photographers to be able to capture location data in their images.

This location data becomes part of the EXIF data and can be used by any number of creative applications. Perhaps the best execution of this so far has been in iPhoto 09 but services such as Flickr also support geo tagging and can display your photos visually on a map.

There are several point and shoot cameras out there offering GPS embedded data into images for the dSLR crowd there is not inbuilt function as yet. To provide this functionality you need an external GPS device that records the location data from the GPS at the time of exposure. Some systems such as the Sony GPS system rely on you matching the time from your images with the time a GPS reading was taken in order to position you at the location the photo was taken. Nikon, provides a better and more integrated solution.